Portable communications receivers, such as pagers, have utilized numerous antenna designs for signal reception. The antenna configuration utilized is a function of performance and space or size requirements. One example of an antenna that provided excellent antenna performance within certain frequency ranges while minimizing size requirements is shown in FIGS. 1 and 2. As shown in these figures, the antenna developed had a small loop antenna which enclosed the entire receiver circuitry. Additional features provided were cosmetic appeal and a rugged means of clip attachment in a minimum amount of space. The loop antenna shown in FIGS. 1 and 2 is described in detail in U.S. Pat. No. 3,736,591 to Rennels et al. entitled "Receiving Antenna for Miniature Radio Receiver", which is assigned to the assignee of the present invention.
While the performance of the small loop antenna of Rennels et al. has been excellent when utilized within the frequency range of 148 to 174 MHz, the antenna performance is substantially reduced when the antenna is utilized at higher frequencies, such as in the UHF frequency range from 450 to 512 MHz.
It has been discovered that since one end of the small loop antenna has been terminated at the ground potential, coupling existed between the antenna to the enclosed receiver ground plane and components of the receiver which were also grounded via stray capacitance. This stray capacitance has been distributed along the complete length of the loop and can effectively short out sections of the antenna at high frequencies, thereby substantially degrading the antenna's performance. Several attempts have been made to overcome this problem, achieving only limited success.
One attempt is shown in FIG. 3. FIG. 3 was previously used in a Pageboy II paging receiver manufactured under Motorola's designation A04FNC2468AN. In this case, the end of the loop antenna which was previously grounded was disconnected or floated from receiver ground, terminating the antenna at a potential other than ground. The resultant stray capacitance provided the return path for the loop antenna. While an improvement in sensitivity was obtainable for the particular antenna configuration, it was noted that as the antenna was brought closer to the ground plane, or as the size of the antenna loop was reduced, the improvement obtained was correspondingly reduced. Consequently, it was possible to obtain little to no improvement in antenna sensitivity compared to grounding one end of the loop antenna when the antenna was a small loop size and/or was in close proximity to the ground plane.
Another attempt to solve this problem was disclosed in U.S. Pat. No. 4,491,978 to Nagata entitled "Portable Radio Receiver with High Antenna Gain." As shown in FIG. 4 from Nagata, the loop antenna and high conversion circuits were isolated from the balance of the receiver by use of high impedance elements Z placed in the ground, power supply and signal lines. It was indicated that an increase in antenna gain was obtainable. However, this solution required three components to obtain an improvement, and the effect of circuit layout on achieving the improvement was indeterminate.